It is often desirable to convert electrical energy sources from some arbitrary voltage to another arbitrary voltage or current. With high enough input voltages, simple linear regulation is often adequate, but often wastes power. For higher efficiency, and for voltages or compliances higher than the input voltage, either the input energy must be AC, or if DC, it must be chopped into AC.
In addition to input variations that must be regulated out, the apparatus for such power conversion usually introduces additional inaccuracies to their output voltages, and usually adds noise related to a chopping or to a mains frequency. This noise usually has fundamental, lower harmonic, and sub-harmonic components usually called “ripple,” and higher harmonic components related to the edge-rate of chopping that cause Electro Magnetic Interference, or EMI.
Regulators are also usually required to maintain their outputs constant despite changes of their loads. For such applications as Voltage Regulation Modules, or VRM's, for computers, regulators may be required to respond to both desired output voltage changes and to load changes from near-no-load to near-full-load in microseconds.
Traditionally, large output filter capacitors have absorbed sudden load changes and filtered out ripple, and fairly simple feedback loops have been used to control regulator output voltage or current, and impedance at low frequencies, but large-value capacitors are physically-large, expensive, and resist nimble voltage control.
Most traditional control loops have depended upon significant Effective Series Resistance, or ESR, of filter capacitors to allow into their feedback loops a small amount of high-frequency ripple, which has been applied to lead networks to stabilize their control loops.
Beyond the frequencies where practical gain-bandwidths enable active control of output, the minimum amount of ripple on the output of a switching regulator is set by the equation dV=l*dT/C, where V is the output voltage in volts, I is the load current in amperes, T is the time in seconds, and C is the filter capacitor in farads. A good regulator would produce only the amount of ripple indicated by that equation.
Unfortunately, ESR related ripple contains two additional ripple components ESR*Im and ESR*Il where, ESR is that of the filter capacitor, Im is the stroke of current that replenishes the energy of the capacitor, and Il is the load current. Unless the complexity of post-filtration is added, both the additional ripple and EMI of ESR*Im ripple pass out of the regulator. Prior art regulators that require ESR for their stability often incur the expense and complexity of additional filtration to abate EMI.
Recently, relatively large monolithic ceramic capacitors with low ESR have become common. Such near-ideal capacitors can, in principle, reduce ripple to the theoretical, reducing post filtration for abatement of EMI up to above their self-resonant frequencies where they appear inductive. However, the practical application of these near-ideal filter capacitors has been troublesome. They form at the regulator output a near-ideal pole that many prior art regulators have difficulty compensating without compromise.
Some earlier regulator designs simply oscillate unless these capacitors are degraded by adding series resistance. Some regulators may be stabilized by an additional pole significantly lower in frequency than the output pole, but with loss of transient response. Some compensation schemes suppress enough of the wrinkles of their Bode plots to achieve some stability together with decent transient response, but the range of inputs and load over which they are absolutely stable is often limited, and outside that range they often produce sub-harmonic ripple tones that make their ripple larger than the theoretical minimum. Some relief has been afforded by the addition of ramp waveforms to regulator control loops. Many of these prior-art solutions lack universality of application, requiring strict application rules to be followed, or the expense of a custom application design to be incurred. One prior-art solution for stabilizing regulators involves adjustment of a “tuning” resistor to the intended application.
It is also common for the stability and transient response of earlier regulators to be predicated upon a fixed, known, and stable filter capacitance. Many modern loads include unknown capacitance, creating an application difficulty for regulators that are capacitance-sensitive, a problem that is exacerbated by the recent practice of “hot-swapping.”
The advent of low ESR filter capacitance has engendered another problem that for buck-converters may be as severe as energy-balance problems. The L-C filters of such converters often constitute lumped-element transmission-lines that were in times past substantially terminated by the ESR's of filter capacitors. With low ESR capacitors, resonances and reflections may occur within these transmission-line sections, upsetting loop stability and degrading transient response
The problems cited above are occasioned by the prior-art practice of attempting to regulate voltage without addressing the stored energy of regulator components.